Highly refractive molybdenum bodies



United States Patent HIGHLY REFRACTIV E MOLYBDENUM BODIES Leonard; F. Yntema, Waukegan, Ill.,.. and .Edward .A.. Beidl'er,. Columbus, and Ivor E. Campbell, Gahanna,.

Ohio, assignors, by. direct. and mesne assignments, to Fansteel Metallurgical Corporation, North'ChicagmHL', acorporation of New York" Drawing, Application!Qcto,berp14,;19531 r I sen-lama. 386,134.

This invention relates to metal" bodies resistant to oxidation at high temperatures and more particularly vided-"with-a coating or skin .to renderth'e basermetal;

resi'stantto oxidation at high temperatures.

This-application; is a.,continuation -in-part of our copcnding-applicatiom Serial No. 299 ;216, file'd. July. 16;.

1952; which application is .a continuation-in-part'of applicationSerial- No. 202,412,.filed December 22,1950,

now abandoned':,

' The: refractory metal molybdenumhas highly desirable properties" and characteristics for many purposes. For example,- it-retains its mechanical properties at elevated temperatures and is very; desirablefor use in forming electrical furnace heating elements. However inorder to" prevent: oxidation of the metaljat elevated temperatures itisnecessary to excludeoxygenand a continuous.

flo'w ofhydrogen is maintained over the heated resistance-elements: Molybdenum,.of"course, is desirable for many othergapplicati'ons, such as oiljburner. nozzle's, artillery piece nozzles, rocket nozzles, turbine blades or.

buckets and; other-component .parts of jet engines, igni tioncoils for gasand oil burners and valve seats for internal combustion engines, the foregoing useybeing merely-the best'known;

The" present invention makes possible. the protection ofjallmetals, steel, for example,' but particularlymolybdenum; which are designedfor 'use' in products which.

which" is resistant to oxidation at elevated temperatures:

and which protects the'base or core from oxidation at suclr'temperatures. aqcordancewith'these methods consist of a molybdenum. bfase;orcore havingan integral coating or skin of mo lybdenum-silicon alloysor intermetallic compounds. The

silicon content of the protective coatings or skins varies iiiythe molecular ratio of silicon to refractory metal of from aboutlzl' to about. 3': 1, whichcorresponds to alloys o'r: ii1termetallic compounds containing from about 22.5%" to'about 47% silicon. Although the optimum protection of the molybdenumis obtained with coatings or skins... having'a molecular ratio of silicon to molybdenum of about Z: 1 corresponding to a. silicon content. of about,

37%; coatings 'or skins beyond the composition range also atterd'tsome protection for the molybdenum base...

The coating or skin isformed on the molybdenum base... by passing a mixture ofhydrogen and vapors of'a silicon.

halide such as; silicon tetrachloride or silicon tetrabromide over-the heated molybdenum base.v Where the molybd'enum -baseis maintained at a temperature appreciably The molybdenum. bodies formed in,

Mice

below the melting point of'silicon duringthe deposition, the-depositedusilicon ,does, not alloy or diffuseinto the molybdenum to.anyappreciabledegree, the degree, of: alloying or diliusionbeing directly proportional' to the specific temperature employed. The silicon .coatedkmolybdennm isjtlienheated; to a. higher temperature andmain: tained atsuch h' her; temperature to effect asubstantially, complete alloying offflthe silicon and the surface ,portions ofthe molybdenum base to form the protective. coatingpr skin...,

Where the temperature of themolylpdenum body is maintained at a temperature at least as high as themeltef ing point of silicon during the deposition. of the silicon, there is anirhmecliatej allbying of. the siliconand the, surface portions ofjthe molybdenum, I

These"alloyedgco'atings or skins on molybdenum fur-. nisli "an exceedingly: high resistance to oxidation at ;ele'-.

' vatedtemperafiresf. For example, molybdenum wire having a diameter-of approximately 01020 has alife. of-'; approximately; 16" seconds whenheated to a. temperature of abont 15 00fGrin-air. The-same size, molybdenum w-i're wh'en; provided with a' coating having a thick ness of jabout- 0.00Q32"has";a1 life 'Of"4,000"S6C,O I1dS at the same-temperaturez- The thickness'of the-coatingdoes not-represent the: total thicknessofan-alloy'layer; The total thiCkI1S$ '0ff the alloylayer: is roughly 'aboutdouble the thicknessincrease which is' obtained during the coating operation, Throughout this application the term -thic kness"is used to designate-only thethicknessincreaseeffected by the: coating operations The: protection afiorde'd the-molybdenum'by the alloy: coatings 'may' 'be fur'ther illustrated by reference; to the lives=-' of*=coated molybdenum bars havingdimensions of about 055*" 0.05""x -6 and h'aving- 'coatingsof tl'ieorder of 0i00lS tQ OEOOZ in-thicknessa- The-lives-weremeasured by'maiiitainingthe coated bars at'the I designated temperatures -in 'air'-.--

t Temp rat r s 1 I Life. 0.; 815 1, 500 Inexcessoi 8,000 hours.

1, 090 2, 000 In excessoi 3,000 hours:

1", 3701' 2,600" AverageaboutlOO hours.

1, 650 3.000 Average about 70 .hours,--

1,760 3,200 (1 specimen) 77 hours.

1",- 850; #360 5: Average aboun 24 hours.

y, thezpastsag qfl'an electrical current. therethrough Will.v

fail, ,i n general;; ,near itsmid-point whenamaintainedmt. arelatively low temperature. In such instancesihecenter; portion oiithe. wire, will attain, ahigher temperaturevbecause ,offthe greater heatloss. at,.the.ends of the...wire. which are secured'tn rmiuals-i The second'type of'failure which is most commonly; encountered at the highentemperatures. -is a. failure. which occur a pp tintermediate an area of maximumtemz It is.

perature. and; n. area .015 minimum; temp r t re thus apparent thatthe life of.the;.InOIybdemImthawing he. protect1ve..coa ing,on kinis. nottnecessar y' isoler.

y; d pendent. upon. the.maximumc .ternperatureto. which hebo y, l'fbesubjectedt The principal purpose of the present invention is w provide molybdenum bodies and other metal bodies having a coating or an exterior layer which has an appreciably greater resistance to oxidation at high temperatures than the resistance exhibited by molybdenum-silicon alloy coatings.

A further object of this invention is to provide a high- 1y resistant coating or skin which does 'not exhibit a failure at the intermediate temperature zones.

Another object of our invention is to provide a method of applying to or forming on molybdenum an integrally bonded, highly protective coating or exterior layer which will protect the molybdenum against oxidation at high temperatures.

Other objects and advantages of this invention will become apparent from the description and claims which follow.

The present invention contemplates, in particular, bodies of the refractory metal molybdenum having a coating or exterior layer of molybdenum-silicon-boron alloys or intermetallic compounds, but it includes other metal bodies to which the high temperature, oxidation-resistant coating of the present invention is either formed thereon in situ, as by plating the metal base with molybdenum and then reacting it with silicon and boron, or applied thereto by mechanical means, as by welding, for example. Although we have designated the coatings of the present invention as molybdenum-silicon-boron alloys or intermetallic compounds, the precise nature of the coatings or skins has not been determined. It has been established that in accordance with the methods as described in the aforementioned Campbell et al. applica: tions, alloys or intermetallic compounds are formed between the molybdenum and the deposited silicon. The inclusion of small amounts of boron increases the complexity of the coatings and it appears that the precise nature of the coatings is dependent upon the method employed in preparing the bodies. Successive deposition of silicon and boron or boron and silicon produces a coating or exterior layer having a more complex structure than the coating or skin produced by simultaneous deposition of silicon and boron. We therefore do not intend to limit our invention to a coating consisting solely of true alloys or intermetallic compounds of molybdenumsilicon-boron. Our invention contemplates compositions which may consist of such true tertiary alloys or intermetallic compounds or any other type of alloy or composition including molybdenum, silicon and boron in the approximate proportions stated.

We have discovered that the presence of boron in the molybdenum-silicon coatings or exterior layers as described in the aforementioned co-pending applications vastly increases the protective qualities of the alloy coating and also substantially eliminates the second type of failure described hereinbefore; namely, the failure of the coating at an intermediate temperature zone. The coatings contemplated by our invention contain a small amount of boron, that is, an amount of from about 2% up to about by weight, preferably between about 3% and about 7.5%, based on the weight of the skin or coating.

Boron may be incorporated in our coatings by any desired method. The boron and silicon may be applied to and alloyed with the molybdenum either independently or simultaneously.

The boron may be applied to or alloyed with the molybdenum followed by the application of the silicon in accordance with the aforementioned co-pending applications.

Molybdenum wire having a diameter of about 0.080" and provided with a molybdenum-silicon-boron skin or coating by the deposition of boron on the molybdenum followed by the deposition of silicon has an average life of about 95 hours when maintained at 1700 C. in air. The maximum life exhibited by one of a series of such specimens was 416 hours. Of 20 specimens provided with the boron-containing exterior layer, only two exhibited a failure at an intermediate temperature zone when maintained at 1700 C. in air.

Molybdenum-boron compositions in themselves are of little protective value, the life of such coating or skin having a thickness of 0.003" being about 6 hours at a temperature of 1000 C.

The same size molybdenum wire provided with a molybdenum-silicon alloy skin of about 0.001" in thickness has an average life of about 24 hours when maintained at a temperature of 1700 C. in air. The maximum life exhibited by one of a series of such specimens was as great as 47 hours.

The simultaneous deposition of boron and silicon produces a coating or exterior layer which possesses greater protective qualities. Coatings produced by the simultaneous deposition have exhibited lives in excess of 1,000 hours when maintained at a temperature of 1700 C. in air.

The application or deposition of the boron and silicon may be effected by a vapor deposition process and the temperature of the molybdenum body may be such as to promote the alloying action simultaneously with the depo- 7 the heated molybdenum body. The body is maintained p the deposition of the boron and silicon.

at a temperature sufliciently high to cause a reaction between the halide and the hydrogen allowing the boron or silicon, or boron and silicon to plate out on the molybdenum body. Since it is desired to produce what we have termed an alloy coating of molybdenum, boron and silicon, we prefer to maintain the molybdenum body at a temperature sufficiently high so as to effect an alloying of or a reaction between the deposited materials and the surface portions of the molybdenum simultaneously with Temperatures from about 1400" C. to about 1800 C. are satisfactory. In the independent deposition and alloying of boron and silicon, the boron may be deposited on and alloyed with the molybdenum surface by maintaining the molybdenum at a temperature of about 1600 C. The silicon is subsequently deposited on and alloyed with the molybdenumboron surface at a temperature of about 1800 C. In the simultaneous deposition of boron and silicon a temperature of about 1600" C. is satisfactory.

The protective qualities of the alloy coatings or skins are dependent upon the composition of the coatings or skins. The composition of the coating or skin is in turn dependent upon the composition of the halide-hydrogen mixture from which the boron and silicon are derived. As in the case of the molybdenum-silicon alloys, the proportion of silicon in the coatings of this invention also determines the protective qualities of the coatings or skins. Coatings or skins which exhibit the high protective qualities contain combinations of silicon and molybdenum containing from about 22.5% to about 47% silicon, particularly about 37% silicon, and combined boron in amounts of about 2 to 10% of the total weight of the coatings. Calculated on a weight basis, the coatings or skins contain from about 15% to about 40% silicon, from about 2% to 10%boron, and the balance molybdenum. The preferred coatings contain about 22% silicon, about 5.5% boron, and the balance molybdenum. The amount of boron in the coatings has a direct bearing upon the resistance of the coatings of this invention. In turn, the amount of boron deposited is dependent upon the amount of boron in the halide-hydrogen mixture. The presence of boron in the coatings or skins in amounts exceeding about 10% decreases the life of the coatings. The hydrogen which is employed should be substantially free of water vapor or oxygen. The amount of hydrogen present in the gaseous mixture should be at least sufficient to react with the halides and an excess amount asaatreit e; p fe ly Provided to aid. lr hfi remoyalrof: he acidi vapors which. arenformed.

We have iscovered h t. s tisfacto y: coating mayprepared from atmospheres of gaseous mixtures having molar ratios. between: silicon; tetrachloride. and; boron trichloride of from about 2.511 to about 18: 1 and containing from about 1.5 to 6.0-times the amountof hydrogen theoretically required to reduce thehalides. Coatings which have exhibited optimum resistance to oxidation have been obtained from gaseous mixtures wherein the ratio ofsilicon tetrachloride to borontrichloride is about 4.6:1- by; volume and having a silicongtoboron ratio. of about 11.9:1 by weight. The ratio of the amount of hydrogen in the gaseous mixture to the amount of hydrogen theoretically required to reduce the halides or react with the halides was about 3.611.

In the independent deposition and alloying of boron and silicon with molybdenum, the molybdenum body is positioned in a suitable chamber which is purged with hydrogen. The molybdenum body is then heated to the desired plating temperature by any suitable means. For example, in the coating of wire or rod the wire may be heated to a temperature of about 1600 C., by passing an electric current therethrough. The hydrogen is passed through a boron halide such as boron tribromide. The boron tribromide may be maintained at room temperature and the hydrogen passed through the liquid at the rate of about 800 cc. per minute. The resulting gaseous mixture is then brought into contact with the heated molybdenum wire or rod. Such gaseous flow is maintained for about 5 seconds. The flow of gas is then interrupted and the molybdenum body heated to a temperature of about 1800 C. Hydrogen is passed through a silicon halide such as silicon tetrachloride maintained at room temperature at about the same rate of flow and the flow continued for about 8 minutes. The gas fiow is then interrupted and hydrogen may be slowly passed through the chamber while the coated molybdenum is cooling.

In the simultaneous deposition, the coating atmosphere is prepared by passing hydrogen through a silicon halide such as silicon tetrachloride which may be maintained at about 0 C. at the rate of about 800 cc. per minute, and by passing hydrogen through a boron halide such as boron tribromide maintained at room temperature at the rate of about cc. per minute. The two gas streams are then merged and brought into contact with the heated molybdenum which may be at a temperature of about l600 C. This gas fiow is maintained for about 8 minutes. The gas-vapor mixture is then interrupted and hydrogen passed through the container until the coated molybdenum has cooled.

The maintenance of such flow rates and temperatures for the stated period will produce a coating having a thickness of the order of 0.002 on a molybdenum wire having a diameter of about 0.080. Molybdenum wire of this size provided with the exterior layer containing silicon and boron by such simultaneous deposition and alloying has exhibited a life of 1415 hours at 1700 C. (3100 F.) in air and as high as 134 hours at a temperature of about l900 C. (3450 F.).

Increasing appreciably the boron content of the vaporhydrogen atmosphere and thereby increasing appreciably the boron content of the coating substantially decreases the life of the coating. For example, by doubling the amount of boron trichloride vapors added to the hydrogen-silicon tetrachloride mixture produces a coating on 0.080" molybdenum wire which has a life of about 0.05 hour at 1700 C.

It is apparent from the foregoing description that the presence of boron elfects a substantial increase in the life of the molybdenum-silicon alloy coatings or layers. The boron-containing coating substantially eliminates the common failure of molybdenum-silicon coatings which occurs at intermediate temperature zones. This type of fa lureris; in general: the; type; of: failure; encountered; where the;coatedwmolybdenum;bodyris maintained; at temperatures-10f, about; 1200b: Cato about;.l-400f C. :and

higher-s. The. failurexgenerally occurs-,a at zona'wherea under conditions which result in temperature gradients.

Our invention is not to be limited to the deposition of boron and silicon on a molybdenum base by a chemical reaction, i. e., the reduction of the halides of boron and silicon by hydrogen, since in accordance with our invention the boron and silicon may be deposited by mechanical means, as by brushing or spraying a suspension of silicon and boron powders on the molybdenum base, or by a metallizing operation with the aid of a metal spray gun. No matter how the silicon and boron are deposited, they are caused to react with the molybdenum base by a heat treatment, as described.

Nor is our invention to be limited to the in situ formation of a molybdenum-silicon-boron skin (layer) on a molybdenum base, since the said skin may be preformed and applied either to a molybdenum base or to a base of another metal or alloy such as steel, nickel, titanium, etc., which is to be protected from high temperature oxidation, as by mechanical means such as welding and bonding with a metal.

We claim:

1. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures comprising a molybdenum base having an exterior layer consisting essentially of an alloy or intermetallic compound of molybdenum, silicon and boron, the boron being present in the exterior layer in amounts of from about 2% to about 10%.

2. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures comprising a molybdenum base having an exterior layer consisting essentially of an alloy or intermetallic compound of molydenum, silicon and boron, the exterior layer containing combinations of molybdenum and silicon containing from about 22.5% to about 47% silicon, and combined boron in amounts of from about 2% to about 10%.

3. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures comprising a molybdenum base having an exterior layer consisting essentially of an alloy or intermetallic compound of molybdenum, silican and boron, the boron layer containing combinations of molybdenum and silicon containing about 37% silicon, and combined boron in amounts of from about 2% to about 10%.

4. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures comprising a molybdenum base having an exterior layer consisting essentially of an alloy or intermetallic compound of molybdenum, silicon and boron, the exterior being present in the exterior layer. in amounts of from about 2% to about 10%, and the silicon being present in amounts of from about 15% to about 40%, the balance being molybdenum.

5. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures comprising a molybdenum base having an exterior layer consisting essentially of an alloy or intermetallic compound of molybdenum, silicon and boron, the boron being present in the exterior layer in an amount of about 5.5% and the silicon in an amount of about 22%, the balance being molybdenum.

6. As an article of manufacture, a metal body resist- ,7 ant to oxidation at elevated temperatures comprising a base having an exterior layer consisting essentially of an alloy or intermetallic compound of molybdenum, silicon and boron, the exterior layer containing combinations of molybdenum and silicon containing from about 225% to about 47% silicon, and combined boron in amounts of from about 2% to about 10% boron.

7. As an article of manufacture, a metal body resistant to oxidation at elevated temperatures comprising a metal base having an exterior layer consisting essentially 1 of an alloy or intermetallic compound of molybdenum, silicon and boron, the boron being present in the exterior 8 layer in amounts of from about 2% to about 10% and the silicon being present in amounts from about 15% to about 40%, the balance being molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS 1,718,563 Kelley June 25, 1929 1,853,370 Marshall Apr. 12, 1932 2,307,005 Ruben Dec. 29, 1942 2,650,903 Garrison Sept. 1, 1953 2,690,409 Wainer Sept. 28, 1954 U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE 0F CORRECTION February 11, 1958 Patent No 2,823,151

Leonard F Yntema et ale It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Let oers Patent should read as corrected below.

9 same line, for

Column 6, line 54, for "silican read silicon "boron", second occurrence, read exterior line 62, for "exterior" read boron Signed and sealed this 1st day of April 1958.

(SEAL) Attest:

KARL Ho AXLI NE ROBERT C. WATSON Commissioner of Patents Attesting Officer 

1. AS AN ARTICLE OF MANUFACTURE, A REFRACTORY METAL BODY RESISTANT TO OXIDATION AT ELEVATED TEMPERATURES COMPRISING A MOLYBDENUM BASE HAVING AN EXTERIOR LAYER CONSISTING ESSENTIALLY OF AN ALLOY OR INTERMETTALIC COMPOUND OF MOLYBDENUM, SILICON AND BORON, THE BORON BEING PRESENT IN THE EXTERIOR LAYER IN AMOUNTS OF FROM ABOUT 2% TO ABOUT 10%. 